Oxidation, carbon monoxide

Carbon monoxide is the main product of the incomplete combustion in many processes, especially in the FCC units, combustion of exhaust gases, and coke elimination. Coke is oxidized producing CO and CO2 according to the following reactions  

Note that the heat produced during the oxidation reaction of CO is 2.6 times greater than the heat of formation. Therefore, for maximum energy recovering, it requires complete combustion, in particular in the regeneration units. 

Today there is a controversy whether the CO oxidation on supported metals is or not a structure-sensitive reaction. Although some studies have shown that the activity depends on particle sizes below 3 nm and low CO concentrations [20], Cant et al. [21] have not observed any dependence of the turnover frequency with the dispersion on Pt/Si02 catalyst. Sarkany and Gonzalez [22] observed that for some degree of dispersion, this reaction is structure insensitive however, the turnover frequency (TOF) decreased with decreasing dispersion of the Pt/Al203 catalyst. In summary, the degree of structure sensitive depends on concentration and particle sizes on supported catalysts. 

As example, the spent FCC catalysts were tested and compared for different combustion tests. The catalysts were promoted with 300 and 800 ppm Pt and Ce on alumina (220 m /g) and mixed oxides (1-5). Platinum was impregnated with hexachloroplatinic acid (H2PtCl6) and calcined at 500 °C. Then, it was treated 

Boudart M, Djega-Mariadassou G. Kinetics of heterogeneous catalytic reactions. Princeton, NJ Princeton University Press 1984. 

The industrial catalysts currently used for CO oxidation are Hopcalite (mainly MnO, and CuO), but these catalysts are not stable in a water environment, and do not exhibit catal dic activity at around room temperature and deactivate rapidly, being thus unsuitable for long-term use [183,251,263,265]. 

Haruta and co-workers have intensively investigated the ability of gold to catalyse CO oxidation [6,7,31,75,110, 111, 138,139,147,155,183,185,207,261, 262,265-275]. At the time of their early work in the 1980s, where the activity for gold-supported catalysts came as a surprise [183,265], but it is now well known that supported gold nanocatalysts are effective for CO oxidation at very low temperatures, with activity as low as 203 K. Activities do, however, depend on choice of metal oxide support (see Fig. 6.8). 

Significant progress has been made and a number of review articles written [6,7,9,18,74,110,252,276]. It has been shown that Au catalysts have advantages arising from their improved activity at low temperatures and stability in the presence of water [6,7,159]. Supported Pt, Pd, Ru and Rh catalysts exhibited appreciable activity only at temperatures higher than 423 K [277]. Gardner and co-workers have shown that Ag-, Ru- or Pd-based catalysts are less active and deactivate faster than Au supported on MnO and CeOj, [278]. 

Although unsupported powdered gold (mean diameter 76 nm) is active for CO oxidation [138,139], most of the studies made by Haruta and other groups have been on supported gold catalysts, often using Ti02 which is considered a very active support [3,5,31,32,34,43,44,49,62,71,75,99,110,111,137-139,155, 

269-271,273-275,279-297]. However, Au supported on Fe203 can have advantages over Au/Ti02 for commercial applications arising from its lower cost, greater availability and improved catalytic performance, not only for CO oxidation but also for other reactions [70,72,73,110,163,183, 261,262,266,285,298-301]. 

Haruta et al. recognized the high effectiveness of supported gold nanocrystals for the oxidation of CO at very low temperatures, unlike the low activity displayed by other metals [11, 257]. 

It is worth mentioning the importance of the preparation method and the excellent behavior of the a-Fe203 support. Later studies showed that Au/Ti02 was equally effective [257]. 

Electron microscopy studies showed that the most active catalysts contained gold nanoparticles of 2-4 nm in diameter. 

In recent studies, Lahr and Ceyer achieved high activity even at —203 °C, by using an Au/Ni(III) surface this questions the real role of oxide supports [258]. 

The high activity that supported gold catalysts have shown for CO oxidation at ambient temperature makes them ideal candidates for use as respiratory protectors. A copper manganese oxide, Hopcalite, has been used for many years to remove CO in toxic environments. Thus, supported gold catalysts may be chosen in the future. 

However, the analysis of the data was carried out in such a way as to cast doubt on the validity of these conclusions (Vayenas ). Okamoto, Kawamura and Kudo went on to use the e.m.f. interpretation from the above work to further investigate the mechanism of CO oxidation over platinum by using the cell as a probe of the surface coverage of carbon monoxide. 

Earlier work was complemented with exchange current density measure-ments. E.m.f. and current density data were inconsistent with only one charge-transfer reaction involving oxygen. Again, two reversible charge-transfer reactions were considered, (3.3) and (3.4). The anodic branch of (3.3) was neglected as was the cathodic branch of (3.4) based on the approximate values of the e.m.f. of the cell compared to the standard reversible potentials (however. 

Metcalfe and Sundaresan went on to confirm that e.m.f.s obtained in the case of CO oxidation over platinum cannot be explained if reaction (3.3) is the only electrochemical reaction to occur. Even under oxygen rich conditions, carbon monoxide was involved in the electrochemical reactions, the dominant electrochemical reactions being, 

Although only one cathodic reaction was occurring, two anodic reactions were present. 

Under CO rich conditions the behaviour was more complicated due to the involvement of the electrolyte surface through the cathodic reaction. 

The available collision partners M may have different efficiencies in transferring energy to or from the activated complex (Section 9.3.2). Since water is much more efficient than 

After the rapid oxidation that typically occurs in a flame sheet, the temperature is high and the concentration of the O and H radicals may be significant. In the postflame region these radicals react in three-body recombination reactions, mainly 

Reactions such as (R9) serve to release the chemical energy in the radicals as heat, and to maintain the radical levels close to their equilibrium values during cooling of the product gas. 

The mechanism for oxidation of moist carbon monoxide is an extension of the H2-O2 mechanism. Carbon monoxide (CO) is an important intermediate in the oxidation of all hydrocarbons, and an accurate knowledge of the oxidation chemistry of this component is required to obtain a quantitative understanding of the more complex hydrocarbon oxidation processes. For this reason the detailed kinetics of CO oxidation has been the subject of a large number of studies. 

The oxidation mechanism for CO depends on the presence of hydrogen-containing components. In the absence of hydrogen donors, the oxygen atom is the only chain carrier, and CO is oxidized by reaction with O or 02, 

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Stampfl C and Scheffler M 1997 Mechanism of efficient carbon monoxide oxidation at Ru(OOOI) J. Vac. Sci. Technoi. A 15 1635... 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

Emissions from foundry cupolas are relatively small but stiU significant, in some areas. An uncontrolled 2-m cupola can be expected to emit up to 50 kg of dust, fumes, smoke, and oil vapor per hour. Carbon monoxide, oxides of nitrogen, and organic gases may also be expected. Control is... 

Carbon Monoxide Oxidation and Related Reactions on a Highly Divided Nickel Oxide... 

Lithium, promotion of carbon monoxide oxidation, 74, 293 Long range effects, 189... 

Oscillatory reactions carbon monoxide oxidation, 388 electrochemical promotion of, 389 Overpotential activation, 124 anodic, 122 cathodic, 122 cell, 123... 

Carbon Monoxide Oxidation on Platinum Coverage Dependence of the Product Internal Energy... 

Meyer O, HG Schlegel (1983) Biology of aerobic carbon monoxide-oxidizing bacteria. Annu Rev Microbiol 37 277-310. 

Rich JJ, GM King (1998) Carbon monoxide oxidation by bacteria associated with the roots of freshwater macrophytes. Appl Environ Microbiol 64 4939-4943. 

Figure 8. Rate of carbon monoxide oxidation on calcined Pt cube monolayer as a function of temperature [27]. The square root of the SFG intensity as a function of time was fit with a first-order decay function to determine the rate of CO oxidation. Inset is an Arrhenius plot for the determination of the apparent activation energy by both SFG and gas chromatography. Reaction conditions were preadsorbed and 76 Torr O2 (flowing). (Reprinted from Ref. [27], 2006, with permission from American Chemical Society.)...

CARBON MONOXIDE OXIDATION 6.2.1 Carbon Monoxide Oxidation on Platinum... 

Garcia G, Koper MTM. 2008. Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution. Phys Chem Chem Phys 10 3802-3811. 

Watanabe M, Shibata M, Motoo S. 1985. Electrocatalysis hy ad-atoms. PartXn. Enhancement of carbon monoxide oxidation on platinum electrodes by oxygen adsorbing ad-atoms (Ge, Sn, Pb, As, Sb and Bi). J Electroanal Chem 187 161-174. 

Brankovic SR, Marinkovic NS, Wang JX, Adzic RR. 2002b. Carbon monoxide oxidation on bare and Pt-modified Ru(lOlO) and Ru(OOOl) single crystal electrodes. J Electroanal Chem 532 57-66. 

Yoshimi K, Song MB, Ito M. 1996. Carbon monoxide oxidation on a Pt(lll) electrode studied by in-situ IRAS and STM Coadsorption of CO with water on Pt(lll). Surf Sci 368 389-395. 

Carbon corrosion, 300 Carbon monoxide adsorption, 248,250,255, 325-327, 347, 386-391,528-532 Carbon monoxide oxidation... 

Tafel plots and Tafel slopes Eor carbon monoxide oxidation, 164-166, 175... 

Zheng X-C, Wu S-Fl, Wang S-P, Wang S-R, Zheng S-M, Huang W-P (2005) The preparation and catalytic behavior of copper-cerium oxide catalysts for low-temperature carbon monoxide oxidation. Appl Catal A 283(l-2) 217-223... 

Selective transformations Selective styrene ring opening [103] One-pot domino process for regioselective synthesis of a-carbonyl furans [104] Tandem process for synthesis of quinoxalines [105] Atmospheric oxidation of toluene [106] Cyclohexane oxidation [107] Synthesis of imines from alcohols [108] Synthesis of 2-aminodiphenylamine [109] 9H-Fluorene oxidation [110] Dehydrogenation of ethane in the presence of C02 [111] Decomposition of methane [112] Carbon monoxide oxidation [113]... 

Fig. 33. Reaction yield as a function of time for carbon monoxide oxidation at room temperature on pure and doped nickel oxides. NiO (200), A NiO(Li) (250), O NiO (250) X NiO(Ga) (250). Reprinted from (8) with permission. Copyright 1969 by Academic Press, Inc., New York.

IX yer, S. M. Transient infrared studies of carbon monoxide oxidation on a supported platinum catalyst. M. S. Thesis, University of Connecticut, 1980. 

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